The invention relates to a movable reflecting mirror for use in a single lens reflex camera, and more particularly, to a movable reflecting mirror having an area which permits part of light from an object being photographed to be transmitted therethrough.
As is well recognized, a movable reflecting mirror which is used in a single lens reflex camera is provided with a transmitting area through which part of light from an object being photographed passes for reflection by an auxiliary total reflection mirror to be incident on a light receiving element which is provided for purpose of photometry or distance measurement. An example of such construction is illustrated in FIG. 1. Specifically, referring to FIG. 1, a movable reflecting mirror 1 is centrally formed with a transmitting area 1a which is defined by a half mirror. An auxiliary total reflection mirror 2 is disposed on the rear side of the transmitting area 1a and is arranged to be angularly movable. The angular movement of the auxiliary mirror 2 is interlocked with a movement of the movable reflecting mirror 1. Thus, before a shutter release, the movable reflecting mirror 1 is in its down position where it forms an angle of 45.degree. within the path of picture-taking light, and the auxiliary mirror 2 is positioned rearwardly of the movable reflecting mirror 1 substantially at right angles thereto. Light from an object being photographed which passes through a picture-taking lens 3 is reflected in the upward direction by the movable reflecting mirror to be introduced into finder optics including a focussing glass 4, pentaprism 5 and eyepiece 6.
On the other hand, the remaining portion of the light from the object being photographed is transmitted through the transmitting area 1a, which is centrally formed in the movable reflecting mirror 1 by a half mirror, for total reflection by the auxiliary mirror 2 to be incident, through a condenser lens 9, on a photometric, light receiving element 10 which is disposed below and outside the path of picture-taking light at a position which is conjugate to the focussing glass 4 and the surface of a film 8. As the movable reflecting mirror 1 is angularly driven upward toward the focussing glass 4 in response to a shutter release operation, the auxiliary mirror 2 also angularly moves in the same direction or in the upward direction so as to close the transmitting area 1a, and both the movable reflecting mirror 1 and the auxiliary mirror 2 come to a rest at their horizontal positions 1A, 2A located outside the path of picture-taking light and which are shown by dash-dot lines where they overlap each other in a horizontal position, thus intercepting any reverse incidence of light from the eyepiece 6 into the path of picture-taking light. Under this condition, light from the object being photographed which passes through the taking lens 3 is focussed onto the film 8 to form a picture thereon. Reference character 1b in FIG. 1 represents a reflector area which is formed by a non-transmitting, reflecting film of known material.
The movable reflecting mirror 1 having the transmitting area 1a defined by a half mirror as mentioned above suffers from the disadvantage that in the region of the transmitting area 1a, light which is transmitted through the transmitting region 1a and is reflected by the rear surface 1c of the mirror becomes superimposed on the light which is reflected toward the finder optics by the front surface of the mirror, thereby producing a ghost or double image in the sight of the finder optics, making it difficult to recognize the image.
The generation of the ghost will be more specifically considered with reference to FIG. 2. In this Figure, S1 represents a central ray of light from an object being photographed which travels through the transmitting area 1a of the movable reflecting mirror 1 to be directed toward the center of the image field of the film 8, and S2 an oblique ray of the light from an object being photographed which passes through the lower end of the transmitting area 1a to be directed toward the center of the image field of the film 8. Representing the refractive index and the thickness of the movable reflecting mirror 1 by .gamma. and t, respectively, the central ray S1 will intersects with the surface of the movable reflecting mirror 1 with an angle of .theta..sub.0 (45.degree. ), and a major proportion of such light will be reflected upwardly, at right angles to the incident ray, to produce a ray S1a while part thereof will impinge upon transmitting area 1a with an angle .theta..sub.1 which is defined by the relationship sin (90.degree.-.theta..sub.0)=.gamma. sin .theta..sub.1 or cos .theta..sub.0 =.gamma. sin .theta..sub.1. Most of such light will transmit through the transmitting area 1a toward the auxiliary mirror 2 (see FIG. 1), but a fraction thereof is reflected by the rear surface 1c of the transmitting area 1a and thence by the front surface thereof to produce a reflected ray S1b which extends in parallel relationship with the reflected ray S1a for incidence upon the finder optics. This causes a ghost image. As will be evident from the drawing, the spacing l.sub.1 between the both rays S1a and S1b as measured on the front surface of the movable reflecting mirror 1 is given as follows: ##EQU1## On the other hand, the oblique ray S2 intersects with the front surface of the movable reflecting mirror 1 with an angle .theta..sub.0 ' (.theta..sub.0 '&lt;.theta..sub.0 =45.degree.), and a substantial portion thereof is reflected upwardly to produce a ray S2a while the remainder impinges upon the transmitting area 1a with an angle .theta..sub.2 defined by the relationship sin (90.degree.-.theta..sub.0 ')=.gamma. sin .theta..sub.2 or cos .theta..sub.0 '=.gamma. sin .theta..sub.2, in the same manner as described above in connection with the central ray S1. A substantial proportion of such impinging ray will transmit through the transmitting area 1a toward the auxiliary mirror 2, but a fraction thereof is reflected by the rear surface 1c and then directed by the front surface of the transmitting area 1a to produce a reflected ray S2b which extends parallel to the ray S2a for incidence upon the finder optics, thus causing a ghost. The spacing l.sub.2 between the reflected rays S2a, S2b as measured on the surface of the movable reflecting mirror 2 is given as follows: ##EQU2## Since the inequality .theta..sub.0 '&lt;.theta..sub.0 =45.degree. applies, it follows that .theta..sub.2 &gt;.theta..sub.1, and hence l.sub.2 =2t..multidot. tan .theta..sub.2 &gt;l.sub.1 =2t.multidot. tan .theta..sub.1.
A movable reflecting mirror of the prior art which is provided with a transmitting area defined by a half mirror suffers from a serious disadvantage of producing a ghost as mentioned above. A variety of proposals have been made to solve this problem. By way of example, Japanese Laid-Open Patent Application No. 119,030/1978 discloses a reflecting mirror assembly in which a transmitting area is defined by a group of totally transmitting, fine pinholes formed therein. While this approach is effective to substantially eliminate the occurrence of ghost, it causes a diffraction of rays which transmit through the pinholes toward an auxiliary mirror. Consequently, while this arrangement presents no problem whatsoever when it is used for photometry, it cannot be used for a measurement of the distance as utilized in an auto-focus arrangement where a sharp image must be formed on a light receiving element. In addition, the formation of the pinholes is a complex and troublesome procedure, resulting in an increased cost. A movable reflecting mirror is also already known having a transmitting area in the form of slits having a certain width so as to avoid the diffraction phenomenon. However, a degree of size is required, which again causes the reflection of light from the rear surface of the transmitting area, making the occurrence of ghost unavoidable.